This invention relates to a method of producing potassium permanganate from manganese dioxide ore and removing impurities from potassium manganate produced from the manganese bearing ore and which manganate is a precursor to permanganate in the production of potassium permanganate. In particular, the invention involves the electrolytic production of permanganate from manganate where the cathode and anode of the electrolytic cell are separated by a cationic membrane which is permeable to monovalent cations and which is substantially impermeable to anions. This separation creates a catholytic region for the production and separation of potassium hydroxide from the anolytic solution and permits a return of potassium hydroxide to the upstream portion of the process directed to the treatment of the manganese dioxide ore and impure precursor manganate.
The manufacture of potassium permanganate (KMnO.sub.4) from manganese dioxide (MnO.sub.2) containing ore requires at least three steps which require considerable amounts of potassium hydroxide. The first of these steps involves the production of potassium manganate VI (K.sub.2 MnO.sub.4) and may also involve the production of an intermediate potassium manganate V (K.sub.3 MnO.sub.4) from the manganese dioxide ore. A second step which requires considerable amounts of aqueous potassium hydroxide is the leaching of the manganate from insoluble ore impurities to obtain a leach solution of manganate. Yet another step which requires aqueous potassium hydroxide as a solvent, is the electrolytic oxidation of the potassium manganate to potassium permanganate. The first and third of these steps are generally represented by the following reactions: EQU (1) MnO.sub.2 +2KOH+1/2O.sub.2 .fwdarw.K.sub.2 MnO.sub.4 +H.sub.2 O EQU (2) K.sub.2 MnO.sub.4 +H.sub.2 O-electrolysis.fwdarw.KMnO.sub.4 +KOH+1/2H.sub.2
The step represented by equation 1 is a high temperature oxidation with air or other oxygen-containing gas to provide an ore oxidation product and is carried out in the presence of relatively concentrated potassium hydroxide solution, such as about 65% to about 90%. A number of methods are known for this oxidation step. It is known to roast a particulate mixture of manganese dioxide ore and potassium hydroxide. The mixture is roasted with air at about 225.degree. C. while intermittently spraying water on the particulate mixture of solids. Another more preferable method is to add the particulate ore to a highly concentrated potassium hydroxide melt and introducing oxygen at elevated temperatures. This oxidation is described in U.S. Pat. Nos. 2,940,821, 2,940,822 and 2,940,823 which patents are fully incorporated by reference as if fully rewritten herein.
After the initial conversion of manganese dioxide to crude potassium manganate which also contains insoluble solids from the ore, the impure potassium manganate is dissolved or leached from the ore oxidation product with dilute aqueous solution of potassium hydroxide to form a manganate leach solution. Again potassium hydroxide is a necessary element in the purification of the manganate and ultimate production of the permanganate.
Equation 2 represents the electrolytic production of permanganate [Fin(VII)] from manganate [Mn(VI)]. While the electrolytic production of permanganate from manganate has been known for a long time, that electrolysis involves a number of known desired main reactions and also side reactions at the cathode and anode where the side reactions at minimum have to be controlled for the economic production of permanganate. The reaction in the electrolytic cell include:
At the anode
Main reaction: MnO.sub.4.sup.2- -e.fwdarw.MnO.sub.4.sup.-
Side reaction: 2OH.sup.- -2e.fwdarw.H.sub.2 O+1/2O.sub.2
At the cathode
Main reaction: H.sub.2 O+e.fwdarw.1/2H.sub.2 +OH.sup.-
Side reaction: MnO.sub.4.sup.- +e.fwdarw.MnO.sub.4.sup.2-
Side reaction: MnO.sub.4.sup.2- +e.fwdarw.MnO.sub.4.sup.3- which partly hydrolyzes to MnO.sub.2.
In prior electrolytic processes for the production of potassium permanganate, potassium permanganate is obtained in the form of crystals from a concentrated aqueous mother liquor. That aqueous mother liquor is a saturated solution of potassium permanganate which also contains relatively large amount of potassium hydroxide as well as alkali soluble impurities. These impurities are predominately potassium carbonate as well as lesser amounts of potassium silicates, aluminates, phosphates, etc. Prior art processes recognized the need for recirculation and conservation of potassium hydroxide used throughout the process of making potassium permanganate.
It is known that in the industrial production of potassium permanganate that only about 40% of the potassium hydroxide in the process streams is actually used to form the end product potassium permanganate. The remaining 60% of the potassium hydroxide is carried through the process in an aqueous solution as a reaction medium. The reuse of this large excess of potassium hydroxide is an important economic factor in the production of potassium permanganate.
As described in U.S. Pat. No. 3,172,830 to Carus, which is incorporated by reference as if fully rewritten herein, the potassium or alkali impurities in the aqueous mother liquor resulting after crystallization of the permanganate from the electrolytic cell can be causticized into potassium hydroxide using Ca(OH).sub.2 or CaO by the reaction K.sub.2 CO.sub.3 +Ca(OH).sub.2 .fwdarw.2KOH+CaCO.sub.3. This "causticization" conserves potassium hydroxide for reuse in the process such as in the leaching step. At least part of the mother liquor is concentrated to conserve potassium hydroxide for use in the initial oxidation of the manganese dioxide ore. That evaporation is energy intensive.
It is an object of the invention to provide for an improved process for the production of potassium permanganate from manganese dioxide containing ore, particularly as a continuous process.
It is another object of the invention to conserve manganese dioxide ore, electricity and potassium hydroxide in the electrolytic production of potassium permanganate.
It is yet another object of the invention to provide for a process for the production of potassium permanganate from manganese dioxide containing ore using potassium hydroxide to produce a manganate intermediate; and further to purify the manganate intermediate prior to electrolysis where potassium hydroxide (1) is made and collected in a catholytic region of an electrolytic cell and (2) is returned for use in the production of the manganate intermediate.
It is yet another object of the invention to use a cationic membrane not only to conserve raw materials in the electrolytic production of potassium permanganate, but also to minimize undesired side reactions in the electrolytic cell to increase the efficiency in the production of permanganate.
Yet another object of the invention is to inhibit the dangerous and potentially explosive mixing of oxygen and hydrogen gases which may be produced during the electrolysis process.
These and other objects of the invention will be recognized after review of the following description and summary of the invention.